Problem: Bethany sells roses and petunias. The expression $3r+2.5p$ gives the cost (in dollars) of $r$ roses and $p$ petunias. What is the cost of $7$ roses and $8$ petunias? $\$$
Answer: Purchasing $7$ roses and $8$ petunias tells us that $r=7$ and $p=8$. Let's substitute $r={7}$ and $p={8}$ into the expression and evaluate: $\begin{aligned} &\phantom{=}3r+2.5p\\\\ &= 3({7})+2.5({8})\\\\ &= 21+20\\\\ &= {41} \end{aligned}$ It costs $\${41}$ to purchase $7$ roses and $8$ petunias.